External-pressure type hollow fiber membrane modules which filtrates from the outside to the inside of the hollow fiber membranes have various merits including the simplicity of the seal structure which separates raw water which has not underdone membrane filtration from filtrate obtained after membrane filtration, ease of operation management, etc. However, the greatest feature thereof resides in that the module can have an exceedingly large filtration membrane area per unit volume of the module. Because of this, the membrane modules are being increasingly applied in recent years to water treatment processes for producing industrial water or tap water from river water, lake water, ground water, seawater, household wastewater, or industrial wastewater.
When raw water is membrane-filtrated using the membrane module, substances which are contained in the raw water and are to be removed, such as suspended substances and organic matters, accumulate on the outside of the membranes to cause a membrane clogging phenomenon. As a result, the filtration resistance of the membranes increases, shortly rendering the filtration impossible. Consequently, a general method for maintaining the membrane filtration performance is to periodically stop the membrane filtration and conduct physical cleaning. Usually, the filtration step and the physical cleaning step are automatically conducted repeatedly.
Examples of the physical cleaning include air scrubbing in which air is introduced into the lower part of the membrane module to oscillate the membranes in water to thereby shake off the suspended substances adherent to the outside of the membranes and back-pressure washing (backwashing) in which water (washing water) such as filtrate is forced into the membranes by pressure in the direction reverse to the filtration direction of the membrane module, i.e., from the hollow portion-side to the outside of the membranes, to remove the suspended substances adherent to the membranes, etc.
A hollow fiber membrane module has a configuration obtained by disposing a hollow fiber membrane bundle formed by bundling about hundreds to tens of thousands of hollow fiber membranes into a casing and fixing both ends of the bundle by casting a potting material. In the case of external-pressure type hollow fiber membrane modules, however, many modules have a structure in which the hollow fiber membranes are open at one end to enable filtrate to be obtained therethrough and have been sealed at the other end with a potting material. Well known as methods for the fixing by casting a potting material are a stationary method and a centrifugal method. The stationary method is a method in which a potting material is supplied with a constant delivery pump or the like from below the hollow fiber membrane bundle and the potting material is cured, while the centrifugal method is a method in which a potting material is moved to the ends of the casing by centrifugal force and cured. In either method, however, the casting resin creeps up along the outer surfaces of the membranes to a height of about several millimeters to several centimeters at the interface between the membranes and the potting material to form an uneven resin boundary. In the case where the hollow fiber membranes in such a state are oscillated during air scrubbing, local stress is imposed on the hollow fiber membranes at the uneven resin boundary, resulting in the concern of membrane rupture.
In order to prevent such a membrane rupture, a contrivance is being made in which after a hollow fiber membrane bundle has been fixed with a potting material for casting, a cushioning material such as a relatively flexible resin, e.g., a silicone resin, is cast again to form a cushioning layer between the potting material and the membrane interface, thereby preventing a membrane rupture from occurring at the uneven resin boundary.
In general, these membrane modules are fabricated from hollow fiber membranes which have been dried in order to facilitate handling and, thereafter, a hydrophilization treatment is conducted for expelling the air present in minute spaces in the pores of the membranes with a hydrophilizing agent such as an alcohol. This hydrophilization treatment is also performed in the case where the hollow fiber membranes have dried as a result of operation or storage. Specifically, known techniques for hydrophilizing an external-pressure type hollow fiber membrane module are a technique in which alcohol-containing liquid is caused to permeate from the outer surfaces of the hollow fiber membranes and the alcohol-containing liquid is taken out from the hollow portion-side, as shown in FIG. 8, and a technique in which alcohol-containing liquid is caused to permeate by pressurizing from the hollow portions of the hollow fiber membranes while keeping the inner and outer surfaces of the hollow fiber membranes in contact with the alcohol-containing liquid, as described in Patent Document 1.